Conventional methods for enriching an oxygen isotope (17O or 18O ) employ a distillation method disclosed in Non-Patent Document 1 which uses nitric oxide (NO) as the raw material (hereafter referred to as the “NO distillation method”), a distillation method which uses water (H2O) as the raw material (hereafter referred to as the “water distillation method”), a distillation method which uses oxygen (O2) as the raw material (hereafter referred to as the “oxygen distillation method”), or a distillation method which uses carbon monoxide (CO) as the raw material (hereafter referred to as the “CO distillation method”) or the like.
Table 1 is a comparison table comparing the NO distillation method, the water distillation method, the oxygen distillation method, and the CO distillation method.
TABLE 1NO distillationWater distillationOxygen distillationCO distillationmethodmethodmethodmethodRaw materialnitric oxidewateroxygencarbon monoxideOperating pressure (bar)10.411Temperature (K)1213509082Relative volatility14N16O/14N18O = 1.04H216O/H218O = 1.00516O2/16O18O = 1.00612C16O/12C18O = 1.006
The relative volatility value corresponds with the separation factor. When the relative volatility is small, the number of theoretical stages necessary to achieve separation and enrichment of an oxygen isotope is approximately proportional to the inverse of (separation factor −1).
Consequently, as illustrated in Table 1, the NO distillation method enables the number of theoretical stages necessary for separation of an oxygen isotope to be reduced to about 1/10 compared with the other distillation methods (specifically the water distillation method, the oxygen distillation method and the CO distillation method).
Accordingly, the NO distillation apparatus can be reduced in size, and the energy required for achieving separation of the oxygen isotope can be reduced.